如何处理数组C ++中的分段错误

Question

I am creating a program that finds the shortest path from one vertex to another that is based upon a set and tables that keep track vertex information as well as the shortest paths from one vertex to another. This is created using an array, NOT a linked list.

//--------------------------------------------------------------------------
// 'updatepaths' uses the newest vertex which was added to the Set to modify
//  the distances of the remaining vertices (if smaller)
//  in addition to the newly added vertex, it uses the Set, the Vertexinfo
//  and the Shortpath tables
//--------------------------------------------------------------------------

void Paths::updatepaths(int addnode)
{    
  if (done.in(addnode)) //done is a set instance, checks if the value is in the set
  {
    for (int i = 0; i<VERTICES; i++)
    {
      if (shortpath[edgedata[addnode].path[i].vertexto].distance > edgedata[addnode].path[i].weight) //HERE IS THE ISSUE
      {
        shortpath[edgedata[addnode].path[i].vertexto].distance = edgedata[addnode].path[i].weight;
        shortpath[edgedata[addnode].path[i].vertexto].via = addnode;            
      }
    }
  }     
}

I realize that the code is quite difficult to read, but it's the only way to compare vertex distances to one another that I can think of -- the issue is that in the if statement, sometimes it will try to compare values that don't exist in the array.

For example, edgedata[addnode].path[0].weight may contain NO VALUE - thus my program throws an access violation (segmentation fault). I tried setting edgedata[addnode].path[i].weight != NULL in the if statement as well as 0, but you cannot use NULL during arithmetic and it won't ever be 0 if it doesn't exist.

How should I make it so that it won't try to compare values that don't exist? Thanks for the help.

Answer 1

If your logic is regularly hitting NULL objects, there may likely be larger design or implementation issues in your code, but the easiest thing to do here to patch over your immediate problem is to use std::array<>::at() , instead of std::array<>::operator[] , and catch the out_of_range exceptions it can generate:

try {
  if (shortpath.at(edgedata.at(addnode).path.at(i).vertexto).distance >
      edgedata.at(addnode).path.at(i).weight)
  {
    shortpath.at(edgedata.at(addnode).path.at(i).vertexto).distance =
      edgedata.at(addnode).path.at(i).weight;
    shortpath.at(edgedata.at(addnode).path.at(i).vertexto).via = addnode;
  }
}
catch (std::out_of_range const &oor) {
  std::cerr << "Out of Range error: " << oor.what() << std::endl;
}

Alternately, you can short-circuit checks in your if statement along these lines (I probably missed a check or two here, so watch out):

if ((edgedata.size() >= addnode)
 && (edgedata[addnode].path.size() >= i)
 && (shortpath.size() >= edgedata[addnode].path[i].vertexto)
 && (shortpath[edgedata[addnode].path[i].vertexto].distance >
    edgedata[addnode].path[i].weight))
{
  shortpath[edgedata[addnode].path[i].vertexto].distance =
    edgedata[addnode].path[i].weight;
  shortpath[edgedata[addnode].path[i].vertexto].via = addnode;            
}

Answer 2

When you write c++ code, you shoud use the c++ coding style firstly. For example, you can use the std::vector instead of array, then you shoud use the iterator to access the element of vector or use vec.at(i) because this two methods can check the boundary exceeded, In c,it has no way to do so